What Kind of Harm Can Come From Soil Organic Matter?

Just because organic matter is so good doesn’t mean that more is always better. Some of the problems with excessive soil organic matter are obvious. For example, residue from a corn crop can choke planting equipment for a following crop. There are more possible problems with soil organic matter that aren’t so obvious, though. Certain plants produce materials that are harmful to other plants. Alternatively, some organic material is released as a plant decomposes that suppresses the growth of other plants. Both of these effects are called allelopathy. Some examples of allelopathy include: materials leached from the bark of black walnut trees suppressing growth of other plants around the base of the tree; Johnsongrass reducing yields in an infested field because of materials released from the roots; and wheat residue reducing yields of a following sorghum crop.

An even more general problem is that many plant residues will actually remove available nutrients from the soil as they decompose. This is illustrated in Figure 3.1 for nitrogen. Nitrogen is often the biggest plant nutrient problem because it is used in large quantities by both desirable plants and other organisms. Plant-available nitrogen is often low under the best of conditions.

Figure 3.1.  An example of temporary nitrogen soil loss due to the incorporation of high C/N ratio residue into the soil.

When a plant residue is incorporated into soil, decomposition begins by the processes described above. At some point, bacteria go into action; and, in using the plant residue for food, these bacteria require more nitrogen than the plant residue can supply. This can be seen most easily by examining the carbon-nitrogen ratio (C/N). This ratio is simply the ratio of the weight of carbon to the weight of nitrogen in a plant material, soil, or any organism. As an example, C/N for sawdust is about 500/1. This means sawdust contains about 500 pounds of carbon for every one pound of nitrogen. In contrast, wheat straw has a C/N of about 80/1, rotten manure about 20/1, and young alfalfa hay about 13/1. The organic matter in the surface soil which we would typically find in Nebraska has a C/N of about 11/1. Bacteria, on the other hand, have C:N values of about 5/1, so they need much more nitrogen than they get from the plant materials they “eat”. What can they do?

One alternative is simply to slow down and not decompose all of the plant residue at once. This is one reason why some plant materials decompose more slowly than others. The bacteria don’t have the nitrogen they need to decompose the residue quickly. Cornstalks (C/N of about 60/1) and wood chips can remain in a soil for months or years for this reason. Besides simply slowing down, however, the bacteria act as very effective scavengers. They “mine” the soil for every bit of available nitrogen that can be found. Most of this available nitrogen is in the form of nitrate and ammonium. The problem arises when the crop being grown needs available nitrogen at the same time. Bacteria are better competitors than crops for the limited nitrogen resource, so the crops do without until the bacteria die and release the nitrogen they have stored inside themselves back into the soil.

This means that most plant materials, but especially those with a high C/N ratio, should not be incorporated into a soil near the time when a desirable plant will also need nitrogen. If incorporated, a temporary nitrogen deficiency will result in the plant. After all, nitrogen was added to the soil when the residue was added in the first place; but it will take weeks or months, depending on the C/N of the residue and some other factors, for the soil to return to a state where the plant will have adequate supplies of available nitrogen. In this time, plant growth may have been so depressed that yield reductions will be substantial.

What can we do to remedy the situation? One obvious answer would be not to put plant residues into the soil. This would not usually be the best answer because of the many positive effects of returning plant residues to the soil. An alternative is to avoid putting plant residues in the soil when plant nitrogen demand is likely to be high. This could involve simply moving time of incorporation to the fall after harvest, rather than in spring immediately before planting. Applying additional nitrogen prior to incorporation would speed up the decomposition of the organic matter. On a smaller scale, “pre-decomposing” the plant residue by composting helps reduce or eliminate the temporary nitrogen deficiency that arises when high C/N materials are incorporated. Composting gives the bacteria time to work on the plant residue and lowers its C/N ratio before the compost is incorporated. This option is probably most practical for a homeowner dealing with yard waste, such as leaves or grass clippings. A good target value for C/N ratio is the same as the soil, 11/1, although incorporating plant material with C/N up to 20/1 will not usually have a serious or long-term effect on available soil nitrogen.